This invention relates to apparatus for monitoring gas pressure levels in an enclosed chamber and more specifically to apparatus for monitoring a pressurized gas used in hybrid air bag systems for motor vehicles.
The use of air bags for drivers as well as passengers in motor vehicles is becoming more and more widespread. Earlier versions of air bag systems made use of pyrotechnic inflators in which a rapid oxidation of sodium azide causes the air bags to inflate in a very short period of time, i.e., approximately 40 to 45 milliseconds. Sodium azide, however, is hazardous in processing. The material is toxic to unprotected workers and is a powerful, unstable explosive during processing.
As a result, a modified air bag inflator is becoming more common. In the modified system, known as a hybrid inflator, a pressurized gas, typically argon, an inert gas, is used in combination with a smaller amount of less hazardous solid propellant to inflate the air bag. Although hybrid systems take slightly longer to inflate a bag, e.g., 50 to 55 milliseconds, and require a heavier steel housing rather than aluminum used for the pyrotechnic versions, substituting argon gas for a solid propellant substantially lowers the cost of the inflator system and is less hazardous to process. In addition, the cost is reduced because the hybrid inflator does not require a sophisticated filtering system which is required in the pyrotechnic version.
An air bag module comprises an inflator, a metal housing and an inflatable bag. The hybrid version also requires a separate electronic circuit including a sensor to monitor the bottle pressure for the stored gas. That is, it is necessary to be able to determine that the pressure of the stored gas is above a selected level to ensure effective deployment of the air bag when called for.
A pressure switch for use as a sensor in a hybrid air bag system is shown and described in U.S. Pat. No. 4,049,935. The switch is placed within a pressure chamber of an air bag system to monitor the pressure level therein and to provide a signal if the pressure in the chamber decreases below a certain level. The switch employs a diaphragm movable between opposite reinforcement sections. Movement of the diaphragm caused by a pressure change pushes a contact arm against the bias of a spring, through a pin, to engage a stationary contact and complete an electric circuit. As noted in the patent the reinforcement sections are spaced apart on the central axis about 0.035 inches with the diaphragm moving between the two extremities. This distance provides sufficient travel to allow the use of a spring and movable contact arm having sufficient stiffness relative to the diaphragm force to avoid problems caused by vibration. That is, the components of the switch must be chosen so that reliable operation is obtained in the event that the pressure in the chamber decreases below the specified level while at the same time avoiding nuisance tripping or chatter caused by the components of the system vibrating at their natural frequency.
In air bag systems currently being used space occupied by the system is at a premium, particularly with respect to the driver's side. As a result, the space available for the sensor switch has been decreased so that it is very difficult to provide a switch of the type shown in the patent which, due to the required smaller diaphragm and its concomitant reduced travel, has sufficient stiffness to avoid such problems caused by vibration.
It is an object of the present invention to provide a sensor switch which is sufficiently small in size to be useful as an air bag sensor switch. Another object is the provision of a switch which is inexpensive, long lasting and easily calibratable. Other objects, features and advantages of the invention will be apparent from the following detailed disclosure, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts.